The average person who hasn't built any base in size at all will also be more likely to just look sickly if they try to hit 5% body fay before gaining any muscle at all. Further, many people find they gain muscle less efficiently if they are much below 10% body fat on a regular basis (obviously genetics matter here).
Why would the guy who grows more optimally at "12%" worry about hitting 8% unless they already have the size they are after?
Yeah, and the problem there is personal trainers taking SOME info and turning it into some entire plane of reference for what they have to sell.
Yes, the guy trying to gain at 30% body fat or more may have much poorer insulin sensitivity than someone who maintains 15% or less. Trying to turn that into "the leaner you are the better your insulin sensitivity" is very much an oversimplification.